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Glow Plug Warning Lamp Not Working


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I am a total newbie to LR - I have just bought a series III 4 weeks ago - 2 weeks have been off the road with blown head gasket.

Just got everything running again - so I am now trying to resolve a few electrical niggles - the one I am working on now is the glow plug warning lamp not working.

Glow plugs are working fine

Bulb is ok

Wire continuity seems ok

I believe the previous owner told me it has the defender glow plug upgrade in parallel.

Any idea as the first place to start looking for the fault please



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OK, we will assume it's the original 2.25 diesel, in which case the original heater plug circuit comprised 4 plugs AND a ballast resistor, all wired in series. The warning light was connected in parallel with the ballast resistor. In this arrangement each plug got about 1.7 volts, so that's about 7 volts for the plugs, the other 5 being dissipated by the ballast resistor, and also powering the 6v warning light.

If someone has fitted the parallel plug conversion to the 2.25 engine then all plugs get full voltage, and there is no need for the ballast resistor, so the 'source' of the nominal 6 volts to power the bulb has disappeared.

You should find the bulb holder is insulated from the casing, and has two wires. Now you change the bulb to a standard 12v 2.2w MES bulb, as used to illuminate the instruments, and also used for the other warning lights. You connect one of the wires to Earth, and the other to the single wire that is daisy chained across the tops of the heater plugs.
Let's just make this clear, if it is a parallel plug installation there is a single wire connected to all four plugs. In a series installation there are two wires, connected to different places on each plug.

The other point, for the future, is that the parallel plugs for the 2.25 are different to the parallel plugs used in any of the later 2.5 engines, whether they are known as 2.5, 200TDi, or 300TDi

A good number for the required plugs is NGK Y-403T. Obviously other makes are available, but if you start looking for that number you should get the correct physical design (11v probe plugs with M14 thread, 12A each.). A wise virgin would power them via a relay, not directly from the ignition switch, which is how the originals were probably wired.


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Thanks for the reply!.....OK yes the plugs have a single wire connected to them - there is only one wire to any single plug.

BUT...there is still the ballast resistor connected in this arrangement - the plugs are definitely connected to the right hand side - which is fed via a solenoid (same as the starter solenoid on a classic mini) on the right hand side - this creates a very audible 'click' when turned to position 2 on the ignition....Maybe this is being used as a sort of relay as you mentioned?

This was the basis of my original query as I have no visible indication or idea of when to fire the engine....i just wait 30 seconds.


I have not chance to check the voltages yet, but will tomorrow





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If both the wire from the solenoid and the wire to the plugs are connected to the right hand side of the ballast resistor then the resistor isn't 'in the circuit', rather it's just that the right hand terminal is being used as a common connection point for both wires, that have to be connected somewhere.
The alternative is to run a new (longer) wire from the solenoid to the plug and connect nothing to the resistor.

I was going to say there wouldn't be any wire connected to the left hand side of the resistor, but there might still be a thin wire which would be one of the feeds to the warning light, the other warning light feed may have also been left connected, to the right side of the resistor.
It's also possible that both thin wires have been left disconnected and possibly wrapped in tape or just turned back. I forget what the wire colours should be.

Unlike more modern diesels, there is no timing circuit involved to switch off the warning light off as an indication you may start the engine. You make up your own timing based on experience; the actual time will depend on engine condition, the ambient air temperature, and how quickly the starter motor can turn the engine. In practice you will find 30 seconds to be probably the longest you need, if the vehicle has been left out in the frost for a week. I suggest start your experimenting at 10 seconds.


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Hi David,

Thats a great help!

The solenoid has 2 nut & stud connections (A and B for the sake of it) & 1 side spade connection - (C)

So Battery + on (A)

Feed (B) to left hand of the ballast resistor

Ignition to spade (C)

Right feed of the ballast resistor to the glow plugs ( on this feed there is a small wire, which I assume is the indicator light?)

Just on a note though....this morning the engine wouldnt start - this was not due to the glow plugs however, but air in the fuel line (just replaced the head gasket - one of the spill rail pipes had a small hole in allowing air in overnight I think)

But as the engine was turning constantly whilst I bled the fuel - the solenoid failed & was extremely hot (2nd in less than a week) - I now do not have any current to the glow plugs - was this due to the fact that the current fried the solenoid as it was engaged for about a minute?

Can I just get rid of the ballast resistor altogether like you said & if so is the present setup safe to use still?

BTW I live in Malta so cold starting isnt really an issue as it doesnt drop much below 10 degrees in the winter....brrrrrrr :o)

Thanks again



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A remote solenoid is only designed to be held operated for a short time, as long as it takes to normally start an engine. By definition a separate  (remote) solenoid is associated with an inertia starter motor, which are fitted to petrol engines. An inertia starter will not normally be engaged for very long, they soon slip of of mesh.

Your diesel will have (should have) a pre-engaged starter, the solenoid of which has two coils, the first takes a lot of current, and will soon burn out, but is powerful enough to throw the pinion into engagement with the flywheel ring gear. At this point the starter and the second coil is energised, this coil takes less current but is still powerful enough to hold the pinion in mesh with the ring gear. That is why you can keep a pre-engaged starter operating for a long time without the solenoid overheating.

You should change the solenoid for a relay with contacts suitable for the 48 amp load. Despite being sometimes described as a 'high current' or 'high power' relay, the relay coil itself will not take a lot of current so will not get hot.

In summary, by holding the remote solenoids operated for 30 seconds each time you power the heater plugs you will have been overloading the solenoids, which is why they have failed (It isn't normal to hold a starter operated for 30 seconds, which is what these solenoids usually control).

You can, if you wish, keep the ballast resistor, simply using one of the terminals as a common connection point. Removing the ballast resistor is simply to tidy up the engine bay by removing an unused component. Whether you keep the ballast resistor or not, what you will have to do is change the remote solenoid to a relay.


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Thankyou so much David - so a 48amp load relay to replace the solenoid, all makes sense now you have explained it.

Is their a standard glow plug relay that I can utilise?...if so can you let me know which pin outs I should use for my conversion & whether I can use one with a timer as in the later cars?


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